Chap. IX.] OP FIRST CROSSES AND OF HYBRIDS. 17 



that species have been endowed with sterility simply 

 to prevent their becoming confounded in nature ? I 

 think not. For why should the sterility be so 

 extremely different in degree, when various species 

 are crossed, all of which we must suppose it would 

 be equally important to keep from blending together ? 

 Why should the degree of sterility be innately variable 

 in the individuals of the same species ? Why should 

 some species cross with facility, and yet produce very 

 sterile hybrids ; and other species cross with extreme 

 difficulty, and yet produce fairly fertile hybrids ? Why 

 should there often be so great a difference in the result 

 of a reciprocal cross between the same two species ? 

 Why, it may even be asked, has the production of 

 hybrids been permitted ? To grant to species the 

 special power of producing hybrids, and then to stop 

 their further propagation by different degrees of sterility, 

 not strictly related to the facility of the first union 

 between their parents, seems a strange arrangement. 



The foregoing rules and facts, on the other hand, 

 appear to me clearly to indicate that the sterility both 

 of first crosses and of hybrids is simply incidental or 

 dependent on unknown differences in their reproductive 

 systems ; the differences being of so peculiar and 

 limited a nature, that, in reciprocal crosses between the 

 same two species, the male sexual element of the one 

 will often freely act on the female sexual element of the 

 other, but not in a reversed direction. It will be 

 advisable to explain a little more fully by an example 

 what I mean by sterility being incidental on other 

 differences, and not a specially endowed quality. As 

 the capacity of one plant to be grafted or budded on 

 another is unimportant for their welfare in a state of 



